Juvenile Idiopathic Arthritis (JIA) is the most common chronic rheumatic disease in children. Affected children suffer from fatigue, pain, tenderness, and loss of mobility of their joints. The combination of JIA, physical inactivity, and the drugs used to treat JIA, often produce muscle and bone loss which can lead to other serious health problems and to long-term disability. So finding ways to preserve body composition in children with JIA is vital. Aerobic exercise is often recommended for JIA but, studies are not definitive in terms of benefit. Studies of adults with rheumatoid arthritis, a disease similar to JIA, show that resistance exercise can improve strength, functional capacity, symptoms, and may even help to improve the disease itself. Resistance exercise has never been studied in JIA. Therefore, we will examine the effects of home-based aerobic versus resistance exercise on measures of body composition, muscle strength, flexibility and range of motion of joints, aerobic capacity, pain, fatigue, functional capacity, quality of life, disease activity, and inflammation. We hypothesize that the children who do resistance exercise will improve more on these measures compared to those who do aerobic exercise. Forty-four JIA patients, ages 10-18 years old, will take part in this 12-week study. This study will advance our knowledge of the effects of exercise therapy for JIA and will be of particular importance since the proposed home-based program is feasible and has the potential to markedly improve the lives of children with JIA.